The present invention relates to N-phosphonomethyliminodiacetic acid ("PMIDA") of the formula (I): ##STR1## and to N-phosphonomethylglycine. Compound I is an important intermediate in the formation of N-phosphonomethylglycine ("glyphosate"), which is a translocated, postemergence, broad spectrum herbicide.
One conventional processes for the preparation of PMIDA starts with iminodiacetic acid (IDA), and requires the involved preparation of that material from its alkali metal salt. More specifically, the IDA moiety is produced by the alkaline hydrolysis of iminodiacetonitrile (IDAN) or by the oxidative dehydrogenation of diethanolamine under alkaline conditions. If sodium hydroxide is used as the base, the product from either of these methods is an aqueous disodium iminodiacetate (DSIDA) solution. This solution is the feedstock used either directly for conversion to PMIDA or to produce IDA for subsequent conversion to PMIDA. In the second case, disodium IDA solution is typically acidified with sulfuric acid or other strong mineral acid and the resulting IDA is separated from sodium sulfate or sodium salt of the strong mineral acid by fractional crystallization. This isolation of IDA by fractional crystallization, which is described in U.S. Pat. No. 3,808,269, requires an evaporative crystallizer for crystallization of anhydrous sodium sulfate, a cooling crystallizer for crystallization of IDA, two centrifuges, solids conveying equipment, and a sodium sulfate dryer, and storage silos or bins for the isolated IDA and dry sodium sulfate. Mother liquors can be recycled but a portion must be purged to remove impurities. An economically and environmentally significant amount of IDA is lost in this purge stream.
Alternatively, IDA can be isolated by ion exchange, as described in U.S. Pat. No. 2,895,989. Dilute hot disodium IDA solution is passed through a column containing a strongly acidic ion exchange resin at a temperature sufficient to prevent crystallization of the IDA acid which is formed in the column. During the ion exchange reactions, hydrogen ion from the resin is exchanged for sodium ion from the disodium IDA. Maximum recovery of IDA occurs when the equivalents of sodium ion in the disodium IDA feed exactly equals the capacity of the resin. The products are a hot solution of IDA acid and exhausted ion exchange resin in the sodium form. A waste stream of dilute sodium chloride is produced which contains only a trace of IDA. The recovery of IDA acid from disodium IDA is &gt;99%. The IDA acid solution, however, is dilute and must be evaporated to produce a concentration useful for conversion to PMIDA. Such a concentration, however, is far above the solubility limit of IDA acid; therefore, IDA acid must be crystallized. To isolate IDA acid from the dilute solution, the processing equipment which is needed includes an evaporator, a cooling crystallizer, centrifuge, solids handling equipment, and storage bin or silo. To control the concentration of impurities which accumulate during recycle of liquor, a small purge stream must be removed.
Another route to PMIDA is from the alkali metal salt of IDA, which, due to limited solubility of co-product alkali metal chloride, leads to very large quantities of effluent, excessively large processing equipment and lower than optimal recovery of PMIDA. Such a process is described in U.S. Pat. Nos. 4,724,103 and 4,775,498. PMIDA crystals are isolated. The mother liquor contains the by-products generated during the reaction and the excess unreacted H.sub.3 PO.sub.3 and formaldehyde, plus all the sodium from the disodium iminodiacetate used, in the form of sodium chloride. This liquor is not a practical recycle stream without evaporation of a large quantity of water to precipitate the NaCl, and is therefore a waste stream. In addition, PCl.sub.3 is conventionally added to a hot alkaline aqueous salt solution of IDA, which results in a violently exothermic reaction that is dangerous and must be carried out in an anaerobic system due to the generation of spontaneously flammable by-products. A build-up of elemental phosphorous in the PCl.sub.3 addition pipe also occurs.
It is therefore an object of the present invention to provide a process for preparing phosphonomethyliminodiacetic acid that eliminates the drawbacks of the prior art processes.
It is a more specific object of the present invention to provide a process for preparing phosphonomethyliminodiacetic acid that minimizes the generation of effluent.
It is yet a further object of the present invention to provide a process for preparing phosphonomethyliminodiacetic acid that maximizes the utilization of valuable raw materials.